1. Field of the Invention
The present invention relates to a thin film transistor array in a liquid crystal panel having a static electricity preventing circuit.
2. Description of Related Art
Among display devices, a Cathode Ray Tube (CRT) is widely used to enhance color realization. The CRT is suitable as a display device incorporated in a TV or a computer monitor because of its high response time. However, since a CRT requires a predetermined distance between an electron gun and a screen, it is relatively thick and heavy. CRTs also have high power consumption. These factors make CRTs unsuitable as portable display devices.
In order to overcome the limitations of CRTs, many simplified display devices have been suggested. Of these suggestions, the liquid crystal display (LCD) is the most practical and has the widest range of applications.
Compared to the CRT, a conventional liquid crystal display has a darker picture and a slower response time. An LCD, however, does not require an electron gun type of a mechanism, and pixels are selectively operated by a driving circuit. Because an LCD is so thin, it is also suitable as a wall type display. Further, an LCD's light weight allows it to be used in a battery operated notebook computer, or in other portable display systems.
FIG. 1 shows the structure of a general liquid crystal display. As shown in FIG. 1, the liquid crystal display includes a display region 12 and data and gate driver ICs 10 and 11 for generating image signals.
FIG. 2 shows a more detailed structure of a general liquid crystal panel. As shown in FIG. 2, a plurality of scanning (gate) lines 14 and a plurality of data lines 16, crossing the gate lines 14, are formed in a matrix configuration on a first substrate 21. At each cross over portion, a pixel electrode 26 and a thin film transistor (TFT) 13 are formed. On a second substrate 22, which faces the first substrate 21, a common electrode 24 and a color filter 23 are formed. Between the first and second substrates 21 and 22, liquid crystal 25 is injected. The common electrode 24 and the pixel electrode 26 separated therefrom by the liquid crystal 25 function as a pixel of the liquid crystal display. On the outer surfaces of the first and second substrates 21 and 22, polarizers 20 are formed to selectively transmit light depending on the arrangement of the liquid crystal 25.
FIG. 3 shows the structure of a thin film transistor usable in a liquid crystal display. As shown in FIG. 3, the thin film transistor includes a gate electrode 30 formed on a substrate 29, an insulating layer 31 formed over the substrate 29, a semiconductor layer 34 formed on the insulating layer 31 over the gate electrode 30, first and second impurity doped semiconductor layer 36a and 36b formed on a first and second side of the semiconductor layer 34, respectively, a source electrode 32 and a drain electrode 33 formed on the first and second impurity doped semiconductor layers 36a and 36b, respectively, second insulating layer 35 formed over the substrate 29, and a pixel electrode 26 formed on a portion of the second insulating layer 29 and contacting the drain electrode 33 via a contact hole in the second insulating layer 35. The gate electrode 30 is formed using a metal such as aluminum, chromium, and molybdenum. The source and drain electrodes 32 and 33 are formed using a metal such as aluminum, chromium, or molybdenum. The gate electrode 30 is connected to the gate line 14 as shown in FIG. 2. The source electrode 32 is connected to the data line 16 shown in FIG. 2, and the drain electrode 33 is connected to the pixel electrode 26. When a scanning pulse (a scanning voltage) is applied to the gate electrode 30 of the TFT through the gate line 14, a data signal on the data line 16 is transmitted from the source electrode 32 to the drain electrode 33 through the semiconductor layer 34.
The data signal received by the source electrode 32 is applied to the pixel electrode 26 so as to establish a voltage (electric potential) difference between the pixel electrode 26 and the common electrode 24. Due to the induced voltage difference, the orientation of the liquid crystal molecules between the pixel electrode 26 and the common electrode 24 changes. Based on the change in orientation of the liquid crystal 25, the light transmittance of the pixel varies to establish a visual difference between the pixel without the data voltage input and the pixel with the data voltage input. Collectively, these pixels, having visual differences, function as a display mechanism for the liquid crystal display.
In the liquid crystal display shown in FIG. 2, the substrate 21, having the pixel electrode 26, is separated from the substrate 22, having the common electrode 24. That is, on the first substrate 21, the TFTs 13 and the pixel electrodes 26 are formed, whereas on the second substrate 20, the common electrode 24 is formed. During the process of forming the first substrate 21, high level static electricity, which can damage the TETs 13, is generated. In order to prevent damage to the TFTs 13 due to static electricity, conventionally a static electricity preventing circuit has been provided for each of the gate and data lines 14 and 16, and one common short line connecting the static electricity preventing circuits is provided on the first substrate 21.
Also, as shown in FIG. 4, in the process of manufacturing the first substrate, a data short bar 42 and a gate short bar 41 are provided to test the operation of the TFT. The data short bar 42 and the gate short bar 41 are shorted through an external short bar 40. The data short bar 42 and the gate short bar 41 are used to transmit a test signal to each of the TFTs 13 formed on the first substrate 21, and test the operation of the TFTs 13. To test the operation of the TFTs 13, a voltage is applied to each of the gate and data short bars 41 and 42 to obtain an output voltage value from the TFTs 13 for determining whether the TFTs 13 are operating normally.
FIG. 5 shows a connection structure for a gate short bar, a data short bar, a gate line and a data line. As shown in FIG. 5, a gate short bar 50 is connected to a plurality of gate lines 55 and a data short bar 51 is connected to a plurality of data lines 56. The gate lines 55 and the data lines 56 are each connected to one end of a plurality of static electricity preventing devices 52. The static electricity preventing circuits 52 are also connected to a common short line 53. The common short line 53 surrounds the area of a substrate in which a plurality of TFTs 54 are formed, and is connected to the other end of the static electricity preventing circuits 52.
FIG. 6 shows, in detail, the circuitry of a static electricity preventing circuit 52. The static electricity preventing circuit 52 is formed of a plurality of transistors and can be incorporated into the thin film transistor array. Specifically, the gate and source of a first transistor 62 and the source of a third transistor 64 are connected to a first terminal 60 of the static electricity preventing circuit 52. The drain of the first transistor 60 is connected to the gate of the third transistor 64 and a source of the second transistor 63. The gate and the drain of the second transistor 63 and the drain of the third transistor 64 are connected to a second terminal 61.
The static electricity preventing circuit 52 prevents damage from occurring due to static electricity generated during the testing or manufacturing of the liquid crystal display panel. For example, when static electricity is generated at a portion of the gate line 55, the static electricity preventing circuit 52 connected to this gate line 55 prevents the TFT 54 from malfunctioning due to a voltage difference between this gate line 55 and neighboring gate lines 55. The static electricity preventing circuit 52 connected to the data line 56 prevents damage of the TFT 54 due to the voltage difference between this data line 56 and adjacent data lines 56 when static electricity is generated at a portion of the data line 56. Further, since the gate lines 55 and the data lines 56 are all connected to the common short line 53, static electricity generated from any portion of the TFT array can be eliminated.
As shown in FIG. 6, when high voltage due to static electricity is applied to a first terminal 60 of the static electricity preventing circuit 52, a first transistor 62 is turned on so as to turn on a third transistor 64. This establishes a common electric potential between the first terminal 60 and a second terminal 61. Furthermore, when high voltage due to static electricity is applied to the second terminal 61, the second transistor 63 is turned on so as to turn on the third transistor 64. Therefore, an equal electric potential is established between the first and second terminals 60 and 61. Since no static electricity is applied to either the first terminal 60 or the second terminal 61 under the proper operation of the liquid crystal display, the first and second transistors 62 and 63 are not activated. By eliminating all but a minute current flow therebetween, the first and second terminals are maintained insulated from each other.
The first, second, and third transistors 62-64 of the static electricity preventing circuit 52 are formed such that a current flows between the first and second terminals 60 and 61 only when a high voltage, much greater than the drive voltages applied to the gate and data lines 55 and 56, is applied to one of the first and second terminals 60 and 61. Therefore, when a drive voltage is applied to the gate or drive lines 55 or 56, the static electricity preventing circuits 52 do not become conductive, and proper testing or operation of the TFTs 54 occurs. When a high static electricity voltage is applied to a gate or data line 55 or 56, however, the corresponding static electricity preventing circuit becomes conductive, and the electric potential difference between conductive lines (e.g., gate and data lines 55 and 56) is eliminated. This prevents the TFTs 54 from being damaged.
That is, when a voltage is applied to the gate and data lines 55 and 56 for testing the operation of the TFTs 54, the static electricity preventing circuits 52 function as insulators so that the gate lines 55 are not affected by the data lines 56. On the other hand, when static electricity is applied to the gate lines 55 or data lines 56, since the static electricity preventing circuits 52 maintain equal electric potential for the gate or data lines 55 or 56, the TFTs 54 formed on the first substrate are protected.
Further, to securely maintain the voltage difference between the common short line 53 and the gate or data lines 55 or 56, a predetermined voltage is applied to the common short line 53.
While each static electricity preventing circuit 52 in the convention liquid crystal display panel functions as an insulating element during the testing of the TFTs 54, the static electricity preventing circuit 52 adds resistance to the testing circuits 50 and 51. This reduces the effectiveness of the device as a testing circuit. Further, due to the voltage difference at the end portions of the static electricity preventing circuit 52, leakage current is generated which deteriorates the picture quality of the liquid crystal display. That is, although the static electricity preventing circuit 52 functions as an insulator due to its high resistance, a very small amount of electricity is still transmitted through the static electricity preventing circuit 52. Therefore, when a voltage is applied to the gate or data lines 55 or 56 for testing the TFT 54, a portion of the current leaks and accurate testing cannot be performed.
For example, when a scanning voltage is applied to the gate line 55 for operating the TFT 54, a portion of the current generated due to the scanning voltage is transmitted to the common short line 53 through the static electricity preventing circuit 52. This current transmitted to the common short line 53 adversely affects the data lines 56 connected thereto by the other static electricity preventing circuits 52. The data signal applied to the data lines 56 also adversely affects the gate lines 55 via the static electricity preventing circuits 52 and the common short line 53. As a result, accurate testing of the TFTs 54 is not performed.
In order to enhance the testability of the TFTs, it has been suggested to form a first static electricity preventing circuit connected to a first short line and a second static electricity preventing circuit connected to a second short line; where the first short line is connected to the gate lines and the second short line is connected to the data lines. (The common short line is separated into the first and second short lines) However, since there exists a different electric potential between the voltage applied to the gate line and the voltage applied to the data line, an electric potential difference exists between the first short line and the second short line. Consequently, the operation of the static electricity preventing circuit is not reliable. Furthermore, since the conventional liquid crystal display requires the application of a predetermined voltage to the common short line or the first and second short lines, an additional driving circuit is required.